Photographic elements for laser scanner imaging are designed to be imaged by electronically-modulated high resolution raster scanners, which scan the film with a very small spot of light from a high intensity source. Examples of high intensity sources include (i) gas lasers, especially argon ion, emitting at 488 nm, helium-neon, emitting at 633 nm, or helium-cadmium, emitting at 442 nm, (ii) near-infrared (NIR) laser diodes, which may emit light in the range 750-1500 nm, and (iii) light-emitting diodes (LED), which may emit in either the visible or NIR range. In all cases, the spot is scanned very rapidly, so that the dwell time on any part of the photographic element is short, typically from 10.sup.-7 to 10.sup.-6 seconds.
Silver halide photographic films usually respond optimally to exposures of duration of from 1 to 100 milliseconds, and tend to perform relatively badly under microsecond exposures, losing up to 1.0 logE in speed and 50% in average contrast. This is due to the phenomenon of high intensity reciprocity failure (HIRF), which also gives rise to related problems, such as:
(i) intermittency effects, which cause multiple superimposed short exposures to have a progressively greater effect as the time interval separating them is increased from microseconds to milliseconds or longer, PA0 (ii) latent image progression, whereby the latent image gives a stronger developed image when there is a delay period, especially of up to 1 hour, between exposure and development, PA0 (iii) unusually high sensitivity to development conditions, e.g. state of exhaustion of the developer.
It is desirable to overcome all these problems by making a photographic element which does not suffer from HIRF and thus responds equally to any given amount of exposure, regardless of how short or fragmented a form in which the exposure may be delivered.
It is known to prepare photographic emulsions containing small quantities of some Group VIII noble metal compounds. For example, U.S. Pat. No. 4,147,542 discloses photographic emulsion containing at least one compound belonging to Group VIII together with particular sensitising dyes. Such dopants are advantageously added during the crystal growth stages of emulsion preparation, i.e. during initial precipitation, and/or during physical ripening of the silver halide crystals. Halide compounds of rhodium and iridium are the dopants most commonly used in this way. When such dopants are incorporated into conventional, negative working photographic emulsions, certain specific photographic effects are obtained, depending on the particular compound employed.
For example, hexachloroiridate complex salts of formula M.sub.3 IrCl.sub.6 or M.sub.2 IrCl.sub.6 (where M is a Group I metal), are incorporated as emulsion dopants with consequent improvement in sensitivity to high intensity exposure, and reduction in the desensitisation usually caused by mechanical stress. This phenomenon is disclosed, for example, in British Patent Nos. 1 527 435 and 1 410 488, U.S. Pat. Nos. 4 126 472 and 3 847 621, German Patent No. DE 3 115 274, and French Patent No. 2 296 204.
The action on silver halide emulsions of halide compounds of rhodium is altogether different. These compounds produce the effect of increasing the contrast of the developed image, together with overall desensitisation of the emulsion. Rhodium doping is disclosed in a number of patents, e.g. rhodium trichloride in British Patent No. 775 197; sodium hexachlororhodate in British Patent No. 1 535 016; potassium hexachlororhodate in British Patent No. 1 395 923; ammonium hexachlororhodate (III) in British Patent No. 2 109 576 and U.S. Pat. No. 3,531,289, and rhodium chloride or trichloride in German Patent Nos. DT 2 632 202A, DE 3 122 921 and Japanese Application No. 74-33781.
In all the above patents, the practical examples of rhodium or iridium doping are exclusively confined to compounds of these metals having only chloride ligands or a majority of chloride ligands, the remaining ligands being water. They are applied in aqueous solution.
It is assumed that the useful effects, of contrast-increase from rhodium, and high intensity sensitisation from iridium, will be obtained irrespective of the type of compound of these metals which is used, i.e. without influence from the type of ligands attached to the rhodium or iridium ions. Thus, these patents predict that any rhodium compound (or any iridium compound, as appropriate to the individual patents) will be suitable for obtaining the useful effect disclosed in the respective patents.
Beck et al (J. Signalaufzeichnungsmaterialen, 1976, 4, p. 131) disclose the use of some rhodium compounds having ligands other than chloride, which were incorporated as dopants into silver bromide and chlorobromide emulsions, these emulsions being coated and examined sensitometrically as primitive emulsions, i.e. without chemical sensitisation. It was found that successive replacement by water of the chloride ligands in the hexachlororhodate complex causes a progressive reduction in the contrast-increasing effect of the rhodium, whilst the compounds [Rh(dipyridyl).sub.2 Cl.sub.2 ]Cl, and [Rh(NH.sub.3).sub.5 Cl]Cl show small contrast increasing effects and [Rh(NH.sub.3).sub.6 ]Cl.sub.3 is inactive, in these emulsions. The preparation of primitive emulsions containing potassium hexacyanorhodate dopant is also disclosed and an increase in image contrast without the usual desensitisation is reported. The authors conclude that the "photographic rhodium effect" on the sensitivity and contrast is only observable when at least one halide or pseudo halide ligand is coordinated to rhodium.
Unlike iridium, the conventional rhodium chloride dopants do not protect emulsions against image degradation caused by high intensity exposure. Japanese Application No. 74-33781 discloses that whilst the image contrast of an emulsion doped with rhodium chloride or hexachlororhodate is extremely high in the case of ordinary exposure employing light of comparatively low intensity, it decreases considerably with exposure using a high intensity flash, and the result is as if the effect of the rhodium compound has been lost. For this reason, JA No. 74-33781, and British Patent No. 1 395 923 suggest the use of a mixture of rhodium and iridium dopants, in order to obtain good high contrast images from high intensity exposures.
Tests on an emulsion doped with sodium hexachlororhodate have confirmed that this dopant does not control high intensity reciprocity failure. Higher overall logE exposures are needed to give the same density when a high intensity exposure of 1 microsecond duration is used rather than an exposure in the normal millisecond to 1 second range. This reciprocity failure becomes progressively worse at higher image densities, hence the loss in image contrast with brief, high intensity exposures.
We have now found that HIRF can be significantly reduced or prevented in chemically sensitised negative working silver halide emulsions by a particular class of rhodium dopants.